1. Field of the Invention
The present invention relates to a method for retorting and obtaining hydrocarbons from underground shale deposits. More particulary, the present invention relates to a process for the in-situ laser retorting of hydrocarbons from underground shale deposits.
2. Description of the Prior Art
In the past many methods have been devised for the processing of fossil fuels to recover hydrocarbons values therefrom. One such method as described in U.S. Pat. No. 3,652,447 involves first mining oil shale and placing the oil shale, which is crushed, into an enclosure. A pulsed laser beam is used to heat the bottom layers of the oil shale in the enclosure, and air is drawn into the bed of shale to cause eduction of gaseous hydrocarbons upwardly through the shale into a gas collection space. The rising air and gas heat the upper layers of the bed of shale thereby retorting the entire body of shale. Gaseous products are then withdrawn from the base of the enclosure. While this method is applicable to the retorting of previously mined or recovered oil shale, it cannot be used for the in-situ retorting of oil shale.
Methods have been developed in the past for the in-situ recovery and retorting of underground deposits of oil shale. All of these methods share the following basic steps in which a predetermined pattern of wells is drilled in the oil shale formation, and the formation is fractured to increase the permeability of the shale. Thereafter the shale is ignited at one or more centrally locoated wells. After ignition, compressed air is pumped down into the ignition wells to support combustion processes within the shale formation, and the hot combustion gases are forced through the fractured shale to degrade the solid organic material within the shale to an oil product. The oil produced by the thermolytic degradation process is subsequently recovered through other wells. All of these techniques share the common problems of attaining the desired degree of permeability of the shale within the formation by fracturing the oil shale between previously drilled wells, and of underground ignition and heating of the shale.
In the past, a number of methods have been employed to create a permeable shale bed which include hydraulic fracturing, electrolinking, electropneumatic and electrochemical fracturing and fracturing using conventional explosives. Other techniques have used combinations of these methods of fracturing oil shale. Thus far, it has been necessary to recover the fractured shale in order to simulate in-situ processing in above ground retorts by utilizing natural gas for ignition and recycled gas and air injection to support combustion within the shale bed. Alternatively, as shown in U.S. Pat. No. 3,652,447, a laser beam can be employed to ignite the combustion process.
One method has been developed for the in-situ retorting of shale deposits as disclosed in U.S. Pat. No. 3,696,866. In this method two wellbores are drilled into a shale deposit and an electrode is lowered into each of the wells at a position in the shale bed. A high d.c. voltage is then impressed across the electrodes, which results in the formation of a conducting core in the shale deposit. One of the electrodes is removed from one of the wells, and is replaced by an electrolyte solution to a level above the core and an acid resistant electrode. A high d.c. voltage is then impressed across the pair of electrodes which causes electrolysis and results in the formation of free oxygen where the conducting core intersects the solution. With sufficient voltage, intense heating and arcing occurs in the core of the shale thus resulting in combustion of organic materials. Application of the voltage is continued until the combustion zone has completely penetrated the path between the wellbores. This method has the disadvantage of requiring the use of a high voltage source and of the necessity of having to place an aqueous electrolyte into one of the well bores. Moreover, and acid resistant electrode must be used in the electrolyte solution. Accordingly, a need continues to exist for a simpler method for conducting the in-situ retorting of shale deposits for the eventual recovery of hydrocarbon products.